Jamieson, Fausset and Brown on 2 Samuel 13:2

2. for she was a virgin--Unmarried daughters were kept in close seclusion from the company of men; no strangers, nor even their relatives of the other sex, being permitted to see them without the presence of witnesses. Of course, Amnon must have seen Tamar, for he had conceived a violent passion for her, which, though forbidden by the law (Le 18:11), yet with the sanction of Abraham's example (Ge 20:12), and the common practice in neighboring countries for princes to marry their half sisters, he seems not to have considered an improper connection. But he had no means of making it known to her, and the pain of that disappointment preying upon his mind produced a visible change in his appearance and health.